Users should be able to export individual universes

Instead of always downloading your entire notebook, users should be able to just download a specific universe's content (or some selection of universes' content) instead.

No files, tests, or entry points are named in the issue. Start by locating the existing notebook-wide export flow and determine how universe selection and downloaded content are represented. Done means users can export one universe or a chosen selection without downloading the entire notebook, with behavior covered by the relevant tests.

Written by the indexing model from the issue text.